The greatest numbers of cases so far are occurring in the northern part of the state. Dallas County has confirmed 270 human cases of West Nile disease and 11 deaths this year. Harris County, including Houston, has had 19 cases and three deaths.

If you must go out, wear long sleeves and pants. Light-colored clothing In addition, don't forget insect repellent that contains DEET, Picaridin or oil of lemon eucalyptus. Homeowners should also make sure any flowerpots, water dishes, birdbaths and wading pools are properly drained so they are not breeding grounds for mosquitoes

Health officials are taking the additional precaution of spraying. On Wednesday night, just after federal health officials said Texas is on track for the largest outbreak ever of the illness, Harris County authorities used aerial spraying to cover 63,000 acres in the west and north.

"We are in the midst of one of the worst West Nile virus outbreaks ever seen in the U.S.," Petersen said. As of Aug. 21, the only states not reporting West Nile activity were Alaska, Hawaii and Vermont. About half of the 1,118 reported cases of the disease in humans are from Texas, Petersen said.

Petersen said it was unclear why West Nile infections increased in so many areas this year or why Texas has had such a large outbreak.

"Why is this occurring in Dallas or elsewhere as opposed to Houston or some other cities? And the answer is, We don't really know," Petersen said. "Often times West Nile virus is a very local disease, you can have a lot of cases in one area and it has a lot to do with the local ecology of that area," he said during the briefing.

About one in 150 infected get severe symptoms from West Nile disease, which can lead to imbalance, coma, paralysis and death.

Those who recover may feel fatigued for week or even months after the initial illness, Petersen said.

In response to the outbreak, Texas officials have shortened the turnaround time for West Nile tests from 10 days to two "so we can have better real-time information about the amount of virus that's out there," said David Lakey director of the Department of State Health Services in Texas.

He said spraying is only part of the way to prevent the ongoing spread of the disease. He said Texans need to help out the effort by taking basic precautions.

"If we spray and it rains and we don't take care of the larvae that are out there, the draining of water in individuals' yards, we won't be as successful as we need to be," he said.